When it comes to 9' ceilings, what is the best practice for installing drywall on the walls? Some contractors say to use 9' sheets applied vertically while others prefer to lay horizontally. I guess as long as the mud/taper is top notch, you'd never know the difference. Especially in a horizontal install there would be a 1' joint in the middle.

There are arguments for both. If I'm installing it myself (and I swore to myself I'd never do that again) I go vertical. Most pros prefer horizontal. I just don't like head joints. Easier to hide those at the top of the wall, to me. When I have head joints in the ceiling, I make a butt board by stapling a couple strips of paper tape to 48"x6" scrap plywood to suck up the head joints and provide a space for the taped joint. But I'm weird like that.

I also like starting outside corners of walls with the long factory edge because it hides the corner bead better. But again, I'm weird like that.

But if they are doing a 9' wall horizontally, there is 54" wide drywall. That's preferable to a strip in the middle. 10' high wall, and it's two horizontal joints and I don't know why they wouldn't go vertical, but it's not my profession.